Vulliamy, who appeared as the third prosecution witness, spoke about his observations about the circumstances which led to the Croat-Muslim conflict in Bosnia-Herzegovina in 1993 and 1994.
The International Criminal Tribunal for the former Yugoslavia (ICTY) charges Jadranko Prlic, Bruno Stojic, Valentin Coric, Berislav Pusic, Milivoj Petkovic and Slobodan Praljak with 26 counts of crimes against humanity, violations of the laws and customs of war and grave violations of the 1949 Geneva Conventions which were committed during the said conflict.
They are indicted for the ethnic cleansing of Bosniaks (Bosnian Muslims) from the areas of Prozor, Gornji Vakuf, Jablanica, Mostar, Ljubuski, Stolac, Capljina and Vares.
The witness, who was in Bosnia several times from 1992 to 1996, has so far testified at several ICTY trials.
On Monday he described how in August 1992 the then Bosnian Croat leader, Mate Boban, had told him in the southwestern town of Grude that he could not recognise Sarajevo as the capital and that he could not also accept the Bosnian constitution which he said protected and safeguarded the rights of individuals rather than peoples.
According to the witness, Boban, who was then the President of the Croat Republic of Herzeg-Bosna (Croat-controlled areas of Bosnia-Herzegovina), justified all the operations carried out by the Croat forces as the actions aimed to defend the Croat people.
Vulliamy testified about an ultimatum which the Croat Defence Council (HVO) issued in the southern city of Mostar in 1992 asking Muslims (Bosniaks) to surrender their weapons.
The Briton also described an ethnic cleansing campaign in the town of Prozor.
Vulliamy, who was among the first reporters to have visited the Serb-run concentration camps of Omarska and Trnopolje in 1992, also visited the Croat-run camp in the town of Capljina. He said that he was stricken to see a great number of women being held there and he added that detainees, mostly Serbs, had been kept in poor conditions.
He told the tribunal that Muslims in the Croat-run Dretelj camp had also been kept in extremely bad conditions.